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BEHAVIOR OF STRIP FOOTING ON MULTI LAYERED GEOGRID REINFORCED SAND BED A PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Bachelor of Technology in Civil Engineering By MANAS MOHANTY Department of Civil Engineering National Institute of Technology Rourkela 2007

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Page 1: BEHAVIOR OF STRIP FOOTING ON MULTI LAYERED GEOGRID ... · BEHAVIOR OF STRIP FOOTING ON MULTI LAYERED GEOGRID REINFORCED SAND BED A PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT

BEHAVIOR OF STRIP FOOTING ON MULTI LAYERED GEOGRID REINFORCED SAND BED

A PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

Bachelor of Technology in

Civil Engineering

By

MANAS MOHANTY

Department of Civil Engineering National Institute of Technology

Rourkela 2007

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BEHAVIOR OF STRIP FOOTING ON MULTI LAYERED GEOGRID REINFORCED SAND BED

A PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

Bachelor of Technology in

Civil Engineering

By

MANAS MOHANTY

Under the Guidance of

Dr.C.R. Patra

Department of Civil Engineering National Institute of Technology

Rourkela 2007

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National Institute of Technology Rourkela

CERTIFICATE

This is to certify that the thesis entitled, “Behavior of strip footing on multi-layered

geogrid reinforced sand bed” submitted by Sri Manas Mohanty in partial fulfillment of

the requirements for the award of Bachelor of Technology Degree in Civil Engineering at

the National Institute of Technology, Rourkela (Deemed University) is an authentic work

carried out by him under my supervision and guidance.

To the best of my knowledge, the matter embodied in the thesis has not been submitted to

any other University / Institute for the award of any Degree or Diploma.

Date Dr. C.R. Patra

Dept. of Civil Engineering

National Institute of Technology

Rourkela – 769008

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ACKNOWLEDGEMENTS

During the entire project work, I have received endless help and guidance from my

revered guide, Dr. C.R.Patra, right from deciding the topic till the final presentations. I

wish to express my deep gratitude to him for the support I have received from him, not

only within institute hours but also beyond that.

I am thankful to Dr. K.C.Patra, HOD of Civil engineering Department, NIT Rourkela, for

providing me all the facilities needed for the experimental work.

I would like to express my sincere thanks to the department of civil engineering for

allowing me to access the computers in the computer lab for hours together.

The project could not have been completed without the constant support and technical

inputs that I received from my seniors, Mr.Pratap Ku. Haripal and Mr.Rabinarayan

Behera. Their presence was indispensable.

I am also thankful to the staff members of Geotechnical engineering laboratory for their

assistance and cooperation during the course of experimentation.

I am thankful to my parents and other family members for their whole-hearted support

and constant encouragement towards the fulfillment of the degree.

May, 2007 Manas Mohanty

Roll no: 10301016

Department of Civil Engineering

National Institute of Technology, Rourkela

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CONTENTS

Page No

Abstract i List of Figures ii List of Tables iii Chapter 1 Introduction 1

Chapter 2 Literature Review 3

2.1 Review of literature 4

2.2 Geogrids 14

2.2.1 Types 14

2.2.2 Dimensions 15

2.2.3 Applications 16

2.3 Mechanism of load bearing and failure 17

2.3.1 Unreinforced case 17

2.3.2 Reinforced case 18

2.4 Mechanism of geogrid reinforcement 19

Chapter 3 Experimental Set up and Procedure 22

3.1 Sample collection 23

3.2 Characteristics of sand 23

3.3 Geogrid used 25

3.4 Test tank 26

3.5 Equipments used 26

3.5.1 Static hydraulic loading system 26

3.5.2 LVDT indicator 29

3.5.3 Load cell indicator 29

3.5.4 Model footing 29

3.6 Sample preparation 30

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3.7 Test procedure 30

3.8 Geometric parameters 31

Chapter 4 Results and Discussions 34

Chapter 5 Conclusion 44

Chapter 6 Scope for further studies 46

Chapter 7 References 48

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National Institute of Technology Rourkela

ABSTRACT Soil Reinforcement is an effective and reliable technique for improving the strength and stability of soil. The reinforced soil or mechanically stabilized earth is a compacted soil fill, strengthened by the inclusion of tensile elements like geogrids, geotextiles, metal bars and strips. It is now well established in heavy construction industry for the construction of structures like retaining walls, embankments over soft soil, steep slopes etc. Several papers relating to the evaluation of the ultimate and allowable bearing capacities of shallow foundation supported by geogrid reinforced sand and saturated clay have been published. This thesis pertains to the study of the behavior of centrally loaded strip foundation on multi layered geogrid reinforced sand bed. Laboratory model test results for the ultimate bearing capacity of a strip foundation supported on multi-layered geogrid reinforced sand and subjected to central loading are presented. Only one type of geogrid Tensar BX1100 and one variety of sand at one relative density were used. The ultimate bearing capacities obtained from model tests have been compared with the theory given by Huang and Menq (1997)

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List of Figures Figure no. Page no. Figure 2.1 Biaxial Geogrid 14

Figure 2.2 Uniaxial Geogrid 15

Figure 2.3 Interaction of reinforcements with failure wedge 18

Figure 2.4 Deep footing failure mechanism in reinforced sand

supporting a strip foundation (Huang and Tatsuoka, 1998) 19

Figure 2.5 Wide-slab failure mechanism in reinforced sand supporting a strip 20

foundation (Schlosser et al. 1983)

Figure 2.6 Bearing capacity failure above upper geogrid layer 20

Figure 2.7 Anchorage pull out of geogrids due to deformation 21

Figure 2.8 Tensile failure of geogrids 21

Figure 3.1 Grain size distribution curve 24

Figure 3.2 Pot of shear stress Vs normal stress 25

Figure 4.1 Determination of qu (expt.) for unreinforced case of loading 36

Figure 4.2 Determination of qu (expt.) for reinforced case of loading, N = 2 39

Figure 4.3 Determination of qu (expt.) for reinforced case of loading, N = 3 40

Figure 4.4 Determination of qu (expt.) for reinforced case of loading, N = 4 41

Figure 4.5 Variation of BCRu with d/B 42

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List of Tables Table No. Page no. Table 3.1 Grain size analysis 23

Table 3.1 Direct shear test results 24

Table 4.1 Load intensity Vs Settlement (N =0) 34

Table 4.2 Load intensity Vs Settlement (N =2) 39

Table 4.3 Load intensity Vs Settlement (N =3) 40

Table 4.4 Load intensity Vs Settlement (N =4) 41

Table 4.5 Comparison between experimental and theoretical BCRu 42

Table 4.6 Comparison of experimental values of quR with values calculated

from equation proposed by Huang and Menq, 1997. 43

iii

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INTRODUCTION

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The reinforced soil is the soil in which the metallic, synthetic or geogrids are provided to

improve its engineering behavior. The technique of ground improvement by providing

reinforcement was also in practice in olden days. Babylonians built ziggurats more than three

thousand years ago using the principle of soil reinforcement. A part of the Great Wall of China is

also an example of reinforced soil construction. Basic principles underlying reinforced soil

construction was not completely investigated till Henry Vidal of France who demonstrated its

wide application and developed a rational design procedure. A further modified version of soil

reinforcement was conceived by Lee who suggested a set of design parameters for soil reinforced

structures in 1973.

Binquet and Lee (1975) investigated the mechanism of using reinforced earth slab to improve the

bearing capacity of granular soils. They tested model strip footings on sand reinforced

foundations with wide strips cut from household aluminum foil. An analytical method for

estimating the increased bearing capacity based in the tests was also presented. Fragaszy and

Lawton also used aluminum reinforcing strips and model strip foundations to study the effects of

sand and length of reinforcing strips on bearing capacity.

In this thesis, the results of experimental studies on cohesion less soil, reinforced with geogrids

has been presented. Tests have been conducted with the provision of geogrids in different layers

up to four layers at various spacing and the results have been compared with the results of

unreinforced condition. The experimental values have been compared with the theoretical values

obtained from Huang and Menq equation, 1997.

2

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LITERATURE REVIEW

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2.1 REVIEW OF LITERATURE

Guido et al. (1980) performed a series of laboratory model tests on rectangular and

square footing. They indicated that bearing capacity ratio (BCR) at a settlement of 0.1B

increases rapidly with increasing strip length up to a length of about 0.7B after which it

remains relatively constant. They also found similar conclusions using square sheets of

geogrid to reinforced sand.

Omar et al. (1993) have conducted laboratory model test results for the ultimate bearing

capacity of strip and square foundations on sand reinforced with geogrid layers. Based on

the model test results, the critical depth of reinforcement and the dimensions of the

geogrid layers for mobilizing the maximum bearing capacity ratio have been determined

and compared. From this experiment, they have drawn conclusions that for development

of maximum bearing capacity, the effective depth of reinforcement are 2B for strip

footings and 1.4B for square footings. Further they have observed that maximum width

of reinforcement layers for optimum mobilization of maximum bearing capacity ratio is

8B for strip footings and 4.5B for square footings.

Dash et al. (2001) have presented the laboratory test results of strip footings on geocell

reinforced sand beds with additional planar reinforcement. The test results show that a

layer of planar geogrid placed at the base of the geocell mattress further enhances the

performance of the footings in terms of the load carrying capacity and the stability

against rotation. The beneficial effect of this planar reinforcement layer becomes

negligible at large heights of geocell mattress. From the experiments they have drawn

conclusions that the cumulative beneficial effect of geocell mattress and planar geogrid

layer is found to be maximum for h/B = 2, where h = depth of reinforcement from the

base of footing and B = width of footing. The overall performance improvement reduces

with the reduction in the base friction and interlocking of the encapsulated soil in the

geocell pockets with the sub-grade soil through the aperture openings of basal geogrid.

Mandal and Manjunath (1994) have conducted an extensive program of monotonically

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loaded footings. The study is aimed at investigating the effects of a single layer of

geosynthetics reinforcing material on the improvement of bearing capacity and settlement

characteristics of strip footings under plane strain conditions supported by compacted

sand and also to study the effectiveness of placing the reinforcing layer horizontally and

vertically. The bearing capacity increase due to the use of a geosynthetic layer has been

expressed in terms of a non dimensional bearing capacity ratio (BCR). The study shows

that the BCR could be improved up to 1.8 times when reinforcement is suitably located

relative to the footing. The horizontal reinforcement is found to be more effective in

improving the bearing capacity as compared to the vertical reinforcement.

Shin et al. (2001) have done laboratory test to determine the bearing capacity of strip

footing supported by sand reinforced with multiple layers of geogrid of one type. The

results show that the ratio of the critical depth of reinforcement below the footing w.r.t

the width of footing is about 2. For a given reinforcements depth ratio, the BCR w.r.t

ultimate load increases with the embedment ratio of the foundation.

Dash et al. (2001) presented the results from laboratory model tests on a strip footing

supported by sand reinforced with a geocell mattress. The parameters varied in the testing

program include pattern of geocell formation, pocket size, height and width of geocell

mattress, depth of the top of geocell mattress, tensile stiffness of the geogrids used to

fabricate geocell and the relative density of sand. With the provisions of geocell

reinforcement, failure is not observed even at a settlement equal to 50% of the footing

width and a load as high as 8 times the ultimate bearing capacity of the unreinforced

sand. The performance improvement is significant up to a geocell height equal to 2 times

the width of the footing. Beyond that height, the improvement is marginal. The optimum

width of the geocell layer is around 4 times the footing width at which stage the geocell

would intercept all the potential rupture planes formed in the foundation soil.

Busehehrian and Hataf (2003) have performed tests to investigate the bearing capacity of

circular and ring footings on reinforced sand along with numerical analysis. The effects

5

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of the depth of the first layer of reinforcement, vertical spacing and number of

reinforcement layers on bearing capacity of the footings are investigated. Both the

experimental and numerical studies have indicated that when a single layer of

reinforcement layer is used, there is an optimum reinforcement embedment depth for

which the bearing capacity is greatest. There is also an optimum vertical spacing of

vertical layers for multilayer reinforced sand. The bearing capacity also increases with

increasing number of reinforcement layers, if they are placed within a range of effective

depths. They have drawn the conclusions that for circular footings on reinforced sand, the

maximum bearing capacity occurs at different values of u/D and z/D depending on the

number of reinforcement layers and are not unique as reported by some researchers,

where u = depth of first layer of reinforcement, z = vertical distance between layers, D =

circular footing diameter or outer diameter of ring footing. For ring footings, however it

has been concluded that increasing the number of reinforcement layers leads to the

decrease of the optimum values of z. Also it is concluded that a maximum threshold

exists for the effect of the rigidity of reinforcement and therefore choosing a more rigid

reinforcement does not always lead to better results in terms of BCR for circular and ring

footings on reinforced sand.

Dash et al. (1994) have done model studies on circular footing supported on geocell

reinforced sand underlain by soft clay. The test beds are subjected to monotonic loading

by a rigid circular footing. The influence of width and height of geocell mattress as well

as that of a planar geogrid layer at the base of the geocell mattress on the overall

performance of the system has been systematically studied through a series of tests. The

test results indicate that the provision of geocell reinforcement in the overlaying sand

layer improves the load carrying capacity and reduces the surface heaving of the

foundation bed substantially. The performance improvement increases with increase in

the width of the geocell layer up to b/D = 5, beyond which it is negligible. (Here, b =

width of geocell layer, D = diameter of footing). The overall performance improvement is

significant up to a geocell height of about two times the diameter of the footing and

beyond this, the improvement is marginal.

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Benrabah et al. (1996) have done analytical work and undertaken experiments to explore

the changes in stress distribution for a sand medium reinforced by geomembrane layers.

The three results i.e. 1.from experiments; 2.using the Boussinesq method and 3.using the

fast lagrangian analysis of continua calculation program (FLAC) are established. They

have found that even in the case of loaded reinforced soil, the vertical stress distribution

follows Boussinesq’s formula, especially for high loads and despite the discrete nature of

the experimental medium being used. This may be due to the fact that in the foundation

zone, the soil grains are structured in such a way that they closely approximate a

continuous medium. In the case of loaded reinforced soil, the presence of reinforcing

layers has no bearing on the vertical stress, while the horizontal stress shows a large

increase. The use of FLAC program leads to slightly lower calculated stress values,

however the increase of the horizontal stress can be predicted by FLAC.

Aigen Zhao (1996) has presented the failure criterion for a reinforced soil composite. The

failure criterion of reinforced soil presented here is anisotropic due to inclusion of

geosynthetic reinforcement with preferred direction. The slip line method in relation with

the derived failure criteria can be used for calculating the failure loads of geosynthetic

reinforced soil structures. The stress characteristics of reinforced slopes, retaining walls

and foundations are presented and compared with those without reinforcement. The

inclusion of geosynthetic reinforcement enlarges the plastic failure region in a reinforced

soil structure and significantly increases the load bearing capacity.

Palmeira et al. (1998) have stated that geosynthetics reinforcement can be used to

increase the factor of safety of embankments on soft soils, particularly for shallow

foundation layers. They have presented back analysis of some reinforced embankments

that can be found in literature using stability methods commonly employed. The results

obtained suggest that these simple methods are useful tools for predicting factor of safety

of reinforced embankments when the required input data are available and accurate.

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Lopes and Ladeira (1996) have studied the interaction between well graded gravelly sand

and a uniaxial geogrid. The influence of the confinement pressure, soil density and

displacement rate on the pull out resistance of the geogrid is discussed by analyzing the

results of the pull out tests. The major conclusions of this analysis:-

1. An increase in the confinement stress, soil density or displacement rate increases

he pull out resistance of the geogrid.

2. The increase in the pull out resistance when the confinement stress increases, is

not in proportion to the increase in the normal stress because there is reduction in

the adherence factor.

Chunsik yoo (2001) has presented the results of laboratory model tests on the bearing

capacity behavior of a strip footing supported by a geogrid reinforced earth slope. A wide

range of boundary conditions including unreinforced conditions, are tested by varying

parameters such as geogrid length, number of geogrid layers, vertical spacing and depth

of top most layer of geogrid. He results are then analyzed to establish both qualitative and

quantitative relationships between the bearing capacity and the geogrid parameters. He

has found that for reinforced slope loaded with a footing, the failure zone tends to

become wider and deeper than that for the unreinforced slope. The bearing capacity of a

footing situated on the crest of sloping ground can be significantly increased by the

inclusion of the layers of geogrid as in the level ground.

Zhao et al. (1997) have shown the utility of geogrid reinforcement in stabilizing the weak

subgrade of a liquid retention pond in Nebraska. The results of the field tests indicates

that the multilayer geogrid is capable of providing the subgrade support needed to

achieve the required placement and compaction of a 0.6m thick lagoon clay liner over a

weak subgrade. The test also confirms that without the geogrid, the required compaction

cannot be achieved. The use of multi layer geogrid in this project has eliminated a 0.3m

crushed stone layer which resulted in a significant savings in material and excavation

cost.

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Manjunath and Diwakar (1994) have conducted experimental investigation to determine

the influence on the performance of a strip footing located at and near the crest of a

granular slope fill with the inclusion of reinforcing layer within the body of the fill. The

investigations have been carried out by varying edge distances of the footing for three

different slope angles and for two types of geogrids. Both load settlement behavior and

the ultimate bearing capacity of the footing show considerable improvement by the

inclusion of a reinforcing layer at appropriate location in the body of the sloped fill. The

conclusion drawn from the tests are:

1. The insertion of a geogrid reinforcement layer at suitable location of the sloped

fill will considerably increase the load carrying capacity as well as stability of

footing on slopes.

2. At an edge distance of 5.0B, the ultimate bearing capacity of footing becomes

independent of the slopes.

3. At any given slope angle, the footing on reinforced slope exhibits much higher

load bearing capacity than that of unreinforced slope.

Andrzej sawicki (1998) has proposed a simple model of the elasto-plastic soil reinforced

with the visco-elastic geosynthetics. Two modes of reinforced soil behavior are

considered within the framework of mechanics of composite materials. The first mode

corresponds to the elastic soil (E-V mode) and the second one to plastic soil (P-V mode).

During the E-V mode, the initial stress in reinforcement decreases, causing the

regrouping of micro stresses in the soil down to the stage when a yield condition in the

soil is reached. Then the P-V mode begins during which the stress in reinforcement

remains constant but the macro strains increase due to creep. Here a method of estimating

the initial stresses in rheological soil has been presented.

Gurung (2003) has done a laboratory study on the ensile response of unbound granular

base road pavement model using geosynthetics, this paper explains the structural

response of a pavement base layer under applied tensile forces in a laboratory set up.

Experimental results show higher tensile resistance of geosynthetics included in the

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pavement base and such high tensile responses will significantly influence the behavior

of the pavement surfaces. The tensile peak strength of a pavement reinforced with

geogrid is higher than pavement reinforced with geotextile. The high tensile strength of a

reinforced pavement will have significant increase on the behavior of the pavement

surfaces under traffic loading and environmental movements.

Raymond and Ismail (2003) have shown that, the improvement of bearing capacity of

track, highway and runaway embankments on unbound aggregate can be done using

geogrid. Geogrid reinforcement in unbound aggregate will improve the performance of

the transportation support. They have presented experimental results for three different

construction possibilities of geogrid reinforcement in the unbound aggregate layers. The

aggregate layers are subjected to both repeated loading and static loading. In a uniform

deposit of granular aggregate statically loaded by a surface footing, with the introduction

of a single layer of reinforced material, the beneficial effect on load bearing capacity of

the reinforcement decreases from a depth of 0.0625 times the footing width and is

negligible at a depth of about one half of the footing width.

Alawaji (2000) has presented the settlement and bearing capacity of geogrid reinforced

sand over collapsible soil. The potential benefits of geogrid reinforced sand over

collapsible soil, to control wetting induced settlement was investigated. Model load tests

have been carried out using a circular plate of 100mm diameter and geogrids. The width

and depth of the geogrids have been varied to determine their effects on the collapse

settlements deformation modulus and bearing capacity ratios. The results show that there

is significant difference in structural contribution of the tested geogrid which ranges from

95% reduction in settlement, 200% increase in elastic modulus and 320% increase in

bearing capacity. It is found that efficiency of sand geogrid system is increased with

increasing geogrid width and decreasing geogrid depth with respect to bottom face of the

footing up to a certain range, after that there is no such improvement. For efficient and

economical reinforcement of sand pad over collapsible soil, geogrid of width 4D and

depth 0.1D are recommended. (Here, D = diameter of loaded area)

10

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Das et al. (1998) have conducted laboratory tests to find out the effect of transient loading

over a foundation supported by geogrid reinforced sand. In the test, a square foundation is

used and through out the test one relative density is maintained. In al the tests, the peak

value of the transient load per unit area of the foundation exceeded the ultimate static

bearing capacity of foundation supported by unreinforced sand. The conclusion drawn

this test is that the geogrid reinforcement reduces the settlement due to transient loading.

Patra et al. (2005) have done laboratory tests to determine the ultimate bearing capacity

of embedded strip foundation on geogrid reinforced sand. The results have been

compared with the bearing capacity theory developed by Huang and Menq (1997). Based

in the tests, the following conclusions were drawn:

1. For the same soil, geogrid and configuration, the ultimate bearing capacity

increases with increase in embedment ratio, Df/B.

2. The theoretical relationship for ultimate bearing capacity developed by Huang and

Menq (1997) provides somewhat conservative predictions.

It is recommended that tests of this type be carried out for karstic soils and weak cohesive

soils to evaluate the improvement in bearing capacity, which may be helpful in field

conditions.

Kumar and Saran (2003) have conducted laboratory tests on closely spaced strip and

square footings on geogrid reinforced sand. The study was carried out to evaluate the

effect of spacing between the footings, size of reinforcement and continuous and

discontinuous reinforcement layers on bearing capacity and tilt of closely spaced

footings. The interference effects on bearing capacity and settlement of closely spaced

square footings on geogrid reinforced sand were almost insignificant in comparison to

those on isolated footings on reinforced sand, whereas, a significant increase in the tilt of

adjacent square footings has been observed by providing continuous reinforcement layers

in the foundation soil under the closely spaced square footings. A considerable increase

in bearing capacity, settlement and tilt of adjacent strip footings has been observed by

providing continuous reinforcement layers in the foundation.

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Shin et al. (2002) have conducted small scale laboratory model tests to determine the

ultimate bearing capacity of a strip foundation supported by sand with multiple layers of

geogrid reinforcement. The embedment ratio of the foundation was varied from zero to

0.6. It is found that, for the given reinforcement-depth ratio, the bearing capacity ratio

w.r.t ultimate load increases with embedment. The bearing capacity at limited levels of

settlement is smaller than the value at ultimate load.

The following conclusions were drawn:-

1. The critical reinforcement-depth ratio below the bottom of the foundation (d/B)cr

for deriving the maximum benefit from reinforcement is about 2.

2. For a given reinforcement-depth ratio, u/B, h/B and b/B, the baering capacity ratio

w.r.t to the ultimate load (BCRu) increases with the embedment ratio of the

foundation (Df/B).

Jun Otani et al. (1998) have analyzed the bearing capacity of geosynthetics reinforced

cohesive foundation soil reinforced soil loaded by a flexible strip footing using a rigid

plastic finite element formulation. This method is based on the upper bound theorem of

the theory of plasticity and bearing capacity is obtained as a load factor at the ultimate

limit state. The reinforcing material and the surrounding sand are modeled as a single

composite material with an equivalent cohesion. The underlying soft ground is assumed

to be purely cohesive and hence both the reinforced soil and soft ground are modeled

using the Von-Mises failure criterion. The method of analysis used here is firstly verified

for the case of unreinforced ground by comparison with the Prandtl solution. For the case

of reinforced cohesive foundation ground, a series of analysis are conducted by changing

the depth and length of the embedded geosynthetics as well as its number of layers.

Based on these layers, the reinforcing effect on the bearing capacity of the cohesive

foundation grounds is evaluated. From the analysis, it is concluded that the bearing

capacity of the geosynthetic reinforced foundation ground is increased as the depth and

length of the reinforcement are increased, but there is an optimum depth at which the

maximum reinforcement effect is obtained. There is also an optimum number of

geosynthetics layer. The results obtained here are quantitatively checked against the field

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measurements and a design chart also developed for estimation of bearing capacity of

geosynthetic reinforced foundations on soft ground.

Shin et al. (2001) conducted field tests on land reclaimed from the ocean for the

construction of Incheon International Airport in Korea. The field test arrangement was a

plate load test on a granular pad with and without Geogrid reinforcement. The magnitude

of stress is measured below the granular pad and the boundary surface of stress,σ

distribution is inclined at an angle α with the vertical. They have observed that for all

values of Q/A, the magnitude of α increases substantially, when reinforcements are in

place. The reinforcement layers help redistribute the stress,σ over a larger area and

reduce its intensity. This, in turn, allows the foundation to support a larger load per unit

area for a given settlement level.

Patra et al. (2006) published the results of a limited number of studies for the ultimate

bearing capacity of strip foundation supported by Geogrid-reinforced sand and subjected

to eccentric loading. They varied the eccentricity ratio (e/B) from zero to 0.15 along with

the foundation embedment ratio (Df/B) from zero to 1.

The following conclusions were drawn:-

1. For similar reinforcement conditions, the ratio of the ultimate bearing

capacity of eccentrically loaded foundations to that loaded centrally can be related by a

reduction factor.

2. The reduction factor is a function of Df/B and e/B.

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2.2 GEOGRIDS

1. Geogrids are relatively stiff net like material with large openings called apertures.

2. The apertures are large enough to allow interlocking with the surrounding soil and

rock to perform the function of reinforcement.

3. They are incorporated in the base layers of paved or finished surfaces, or in

surface layers of walls and slopes and provide a stabilizing force within the soil

structure itself.

4. This stabilization occurs as the fill interlocks with the grid. The interlocking effect

is determined by the geogrid strength; mesh size and base materials used.

5. Geogrids are made of high modulus polymer materials like polypropylene (PP) and

high density polyethylene (HDPE) and are prepared by tensile drawing.

2.2.1 Types of Geogrids

1. Biaxial Geogrid

• Manufactured by stretching the punched sheet of polypropylene in 2

orthogonal directions.

• Has high tensile strength and modulus in 2 perpendicular directions.

Fig. 2.1 Biaxial Geogrid

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2. Uniaxial Geogrid

• Manufactured by stretching a punched sheet of extruded high density

polyethylene in one direction.

• Has high tensile strength and modulus in one direction.

Fig. 2.2 Uniaxial Geogrid

2.2.2 Dimensions of Geogrids

• The grid apertures are either square, rectangular or elliptical

• Nominal rib thickness = 0.5 – 1.5 mm

• Junctions thickness = 2.5 – 5.0 mm

• Aperture dimension = 25 – 150 mm

• Open area of Geogrid > 50% of total area

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2.2.3 Applications

Geogrids are typically used for:-

1. Slope Reinforcement: Highway embankments, overpasses, landslide or erosion-

prone surfaces and landfill walls.

2. Base Reinforcement: Foundations for roads, parking lots, railroad track beds,

airport tarmacs and runways.

3. Wall Reinforcement: Retaining walls, airport noise barriers, bridge supports and

sea walls.

4. Berm Reinforcement: Spillway channels for earthen dams, levees and waste

containment ponds.

Till date, several laboratory model tests have been carried out relating to the Load Bearing

Capacity of Shallow Foundations supported by sand reinforced with various materials such

as geogrids, geotextiles, rope fibers, metal strips and bars.

The use of geogrids for soil reinforcement has greatly increased over the last decade

primarily because of following reasons:

1. Geogrids are dimensionally stable.

2. have high tensile modulus (that means low strain at high load)

3. open grid structure (that provides enhanced soil reinforcement interaction)

4. positive shear connection characteristics

5. light weight

6. long service life

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2.3 MECHANISM OF LOAD BEARING AND FAILURE

2.3.1 Unreinforced case

The bearing capacity of foundation soil comes from cohesion factor, c and frictional

factor, Φ. In granular soil (dry sand), load taking factor is only the frictional one. Safe

bearing capacity is defined as the maximum pressure, which the soil can carry safely

without the risk of shear failure. Shear failure may result from the foundation failure as

well as from excessive settlement. Before the application of load, the soil below the base

of the footing is in elastic equilibrium and after the load is applied, the soil passes from

elastic to plastic equilibrium with failure.

The three principal modes of shear failure in soil are:

1. General Shear Failure: - The soil properties are assumed to be such that a slight

downward movement of the footing develops fully plastic zones and the soil

bulges out. It occurs in relatively incompressible soil. Dense sand having relative

density greater than 70% fails under general shear failure.

2. Local Shear Failure: - Large deformations may occur below the footings before

the failure zones are fully developed and is associated with considerable vertical

soil movement before soil bulging takes place. This type of failure may take place

in fairly soft or loose and compacted soil. Loose sand having relative density

between 50-70% fails under local shear failure.

3. Punching Shear Failure: - No lateral movement takes place. When the load is

increased, vertical movement of the footings occur and the soil surrounding the

footing remains relatively in original position i.e. it does not take part in failure.

Hence there will be no tilting of footing and no bulging of surface soil. This type

of failure is expected in loose sand having greater compressibility and relative

density less than 35%.

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2.3.2 Reinforced case

The reinforced soil technique is based on the mobilization of the interfacial shearing

resistance between the soil and reinforcement which in turn restrains the lateral

deformation of the soil.

Soil below a footing consists of three zones, viz. an active Rankine zone

(Zone 1) which moves vertically downwards and displaces the radial Prandtl zone (Zone

2) in a lateral direction and the passive Rankine zone (Zone 3) in upward direction.

Fig 2.3 Interaction of reinforcements with failure wedge

In case of reinforced soil, the above possible failure surface is intercepted by the

horizontally placed reinforcements. Therefore, for the lateral movement of zone 2 to

occur, soil in that zone has to overcome the frictional resistance at the soil-reinforcement

interface. Thus the effect of the reinforcement is to check the lateral flow of soil beneath

the footing by introducing lateral confinement.

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2.4 MECHANISM OF GEOGRID REINFORCEMENT

The provision of geogrid reinforcement imparts anisotropic mechanical properties,

increased stiffness, tensile strengths, increased bearing capacity to the foundation soil.

Reinforcing mechanism comes from the interaction between geogrid and soil in one or

more of the following way.

1. Soil shearing on plane surface of the grids (i.e. skin friction)

2. Soil bearing on lateral surface of grids (i.e. passive pressure resistance of the

contact area between soils and geogrids)

3. Soil shearing over soil through the apertures of the grids (i.e. inter-facial shear on

the surface of a rupture zone created during shearing)

The failure mechanisms are as follows:-

• Huang and Tatsuoka (1988, 1990) proposed “deep foundation mechanism” of

failure. In this case, width of reinforcement, b = width of foundation, B and a

quasi-rigid zone is developed beneath the foundation.

Fig 2.4 Deep footing failure mechanism in reinforced sand supporting a strip foundation

(Huang and Tatsuoka, 1998)

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• Schlosser (1983) proposed “wide slab mechanism” of failure in soil.

In this case, b > B

Fig 2.5 Wide-slab failure mechanism in reinforced sand supporting a strip

foundation (Schlosser et al. 1983)

• Binquet and Lee (1975) proposed a rational design method. According, to this

study, if layers of rein. are placed under a shallow strip foundation, the nature of

failure in soil mass will be as follows :-

1. Shear failure of soil above the uppermost layer of the reinforcement. The mode of

failure is possible if depth to the top most layer of reinforcement is sufficiently large,

i.e. when Bu

≥ 32

Fig 2.6 Bearing capacity failure above upper geogrid layer

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2. Reinforcement pull out failure: This type of failure occurs for reinforcement placed at

shallow depths beneath the footing with sufficient anchorage, i.e. when Bu

< 32

,

N = 2 – 3

Fig 2.7 Anchorage pull out of geogrids due to deformation

3. Reinforcement tension failure: This type of failure occurs in the case of long and

shallow reinforcement for which the frictional pull-out resistance is more than the

tensile strength. The most beneficial effect of reinforced earth is obtained in this case,

i.e. when Bu

<32

, and N > 4 but not more than 6 – 7.

Fig 2.8 Tensile failure of geogrids

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EXPERIMENTAL SET UP AND PROCEDURE

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3.1 SAMPLE COLLECTION

The sand collected from the river bed is made free from foreign matters i.e. roots, organic

matters etc. and is cleaned by washing. Then it is oven dried and properly sieved, passing

through 700 µ and retaining on 300 µ IS sieve. Dry sand is used as soil medium for the

test as it does not include the effect of moisture and hence the apparent cohesion

associated with it. Also due to non-availability of laboratory facilities, the conducting of

test in a complex situation developed due to presence of moisture and cohesion has been

avoided.

3.2 CHARACTERISTICS OF SAND

The characteristics of sand used are as follows:

1. Specific gravity G = 2.618

2. Maximum void ratio emax = 0.995

3. Minimum void ratio emin = 0.664

4. Relative density Id = 72%

5. Dry density γd = 1.49 gm/cc

6. Angle of internal friction at the adopted density Φ = 42.34°

Table 3.1 GRAIN SIZE ANALYSIS

Sl. No. Sieve size (µ) Wt. of sand

retained (g)

% Retained Cumulative

% retained

Cumulative

% finer

1 710 0 0 0 100.0

2 600 132.7 26.54 26.54 73.46

3 500 72.50 14.50 41.04 58.96

4 425 239.8 47.96 89.00 11.00

5 300 55.00 11.00 100.0 0

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Fig 3.1 GRAIN SIZE DISTRIBUTION CURVE

GRAIN SIZE DISTRIBUTION CURVE

0

10

20

30

40

50

60

70

80

90

100

100 1000SIEVE SIZE (MICRON)

CU

MU

LATI

VE %

FIN

ER

Table 3.2 DIRECT SHEAR TEST RESULTS

Sl. No. Normal stress (kg/cm2) Shear stress (kg/cm2)

1 0.315 0.309

2 0.630 0.523

3 0.945 0.811

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Fig 3.2 PLOT OF SHEAR STRESS VS NORMAL STRESS

DIRECT SHEAR TEST

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

NORMAL STRESS (kg/cm2)

SHEA

R S

TRES

S (k

g/cm

2)

c=0, Φ=42.34°

From the above graph; c = 0 and Φ = 42.34°

3.3 GEOGRIDS USED

Tensar BX1100 geogrid is used as reinforcement. The physical and mechanical properties

of the geogrid as listed by the manufacturer are given below:

Polymer polypropylene

Aperture shape rectangle

Aperture size (MD/XD) (mm) 25/33

Rib thickness (mm) 0.75

Junction thickness (mm) 2.80

Tensile strength at 5% strain (KN/m) XD 8.46

Tensile strength at 5% strain (KN/m) MD 13.42

Long term allowable strength in crushed aggregated MD N/A

(N.B. MD – Machine direction, XD – Cross machine direction)

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3.4 TEST TANK

Bearing capacity tests were conducted in a box measuring 100 cm x 37 cm x 65 cm and

made up of mild steel of 8 mm thickness. Scales are fitted on the internal walls of the box

so that it will be helpful in maintaining the required density accurately. The sides of the

box are heavily braced to avoid lateral yielding. The following considerations are taken

into account while deciding the dimension of the box.

• As per provision of IS 1888-1962, the width of the test pit should not be less than

5 times the width of the test plate, so that the failure zones are freely developed

without any interference from sides.

• Chumar (1972) has suggested that in case of cohesion less soil, the maximum

extension of failure zone is 5B to the sides and 3B below the footing.

By adopting the above box size for the model footing (8 cm x 36 cm), it is ensured that

the failure zones are fully and freely developed without any interference from the sides

and bottom of the tank.

3.5 EQUIPMENTS USED

1. Static hydraulic loading system

2. LVDT indicator

3. Load cell indicator

4. Model footing

3.5.1 Static hydraulic loading system

The Hydraulic Pressure Testing Equipment is designed to test concrete and soil samples

at high pressure. The testing pressure can be set from zero to 115 bar to get pressing force

of 10 T in Cylinder - 1 and 20 T in Cylinder - 2. The test piece is kept on the machine

base. Test pieces up to 1000 mm x 1500 mm can be tested.

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The equipment comprises of the following main units.

# Hydraulic System

# Test Stand

# Electric Control Panel

HYDRAULIC SYSTEM

The system comprises of a 150 lit reservoir mounted on which the gear pump is placed

along with the various hydraulic elements. The piston pump is driven by a 3.75 kW/ 5

HP, 1440 rpm AC motor.

The system maximum pressure is 115 bar. The system is provided with standard elements

like pressure relief valve, return line filter, suction filter, pressure gauge with isolation

valves, oil level and temperature indicator, etc.

The system is provided with the following hydraulic elements:-

GEAR PUMP

The gear pump develops the test pressure of 115 bar (max.). The pump gets the oil supply

from the reservoir. The gear pump design pressure is 210 bar against required maximum

system pressure of 115 bar.

ELECTRIC MOTOR

A 3.73 kW/5 HP, 1440 RPM, foot mounted AC motor drives the gear pump. The

direction of rotation of the motor (and the pump) is marked on the motor body. The

motor drives the pump through a geared coupling.

BREATHER

A breather cum oil filling cap is provided on the reservoir for filling of oil and maintain

the inside pressure of the reservoir to atmosphere.

RETURN LINE FILTER

The return line filter filters the hydraulic oil returning to the reservoir from the system

during the operation. A 20 μ filter is provided.

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PRESSURE RELIEF VALVES

A proportionate pressure relief valve is used to control the pressure from zero to the

maximum system pressure of 115 bar. The pressure is regulated by varying the coil

voltage from 0 to 10 V DC through an electronic card.

The pressure required for both the cylinders are the same. Cylinder-1 gives 10 T and

Cylinder-2 gives 20 T at the maximum operating pressure of 115 bar.

A ‘two position’ directional valve is used with the pressure relief valve to load/un-load

the system pressure.

DIRECTIONAL VALVES

Five directional valves are provided, one for the movement of the ram of Cylinder-1,

another for the movement of the ram of Cylinder-2, the third for positioning the Cylinder-

1 (in the horizontal axis), the fourth for positioning of the Cylinder-2 (in the horizontal

direction) and the fifth for taking the platen up and down.

TEST STAND

The Test Stand is of very rigid construction. The machine frame can be placed on

concrete floor for operating. No foundation is required. The test piece is placed on the

machine base. The base can accommodate 1500 mm x 1000 mm test piece. The platen

has four positions which can be set as per the height of the test piece. The Platen is

moved up and down with the help of two hydraulic cylinders actuated by two push

buttons. For taking the platen up or down, the four positioning pins have to be removed

and the two hydraulic cylinders are actuated by operating the push buttons. The 10 T and

the 20 T pressing cylinders move on rollers. They are moved from left to right or right to

left with the corresponding hydraulic cylinders. These positioning cylinders are actuated

by push buttons.

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ELECTRIC PANEL

The Hydraulic Pressure Testing Equipment is designed for semi-automatic operation.

Loading/unloading of the test piece is however is manual. The main motor (hydraulic) is

initially started with a push button. It can be stopped by another push button. A selector

switch is provided for moving the platen up and down, taking the pressing cylinders left

and right and for actuating the 10 T and the 20 T pressing cylinders.

In automatic cycle, the pressing time can be set as required. Also the pressing force can

be programmed with respect to the pressing time. On completion of the pressing cycle the

ram returns back to its ‘home’ position. The control voltage for the solenoid coils (for the

hydraulic and pneumatic systems) is 220 V AC. Control voltage for the digital meters is

24 V DC.

3.5.2 LVDT indicator

Linearly Variable Displacement Transducer indicator is used to measure the settlement of

the footing produced due to application of pressure on the footing. Its accuracy is up to

0.001 mm

3.5.3 Load cell indicator

This instrument is used to indicate the load applied on the footing due to increase in

pressure after regular intervals of time. Accuracy is up to 1 kg for 10 T load cell indicator

and 2 kg for 20 T load cell indicator.

3.5.4 Model footing

Model footing used for laboratory tests is made of mild steel plate of size 8 cm x 36 cm x

2.5 cm. The length of the footing is made almost equal to the width of the tank, in order

to create plane strain conditions within the test arrangement. A cross-mark is made

exactly at centre of the footing for the centric application of load on the footing.

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3.6 SAMPLE PREPARATION

The internal dimensions of the box are measured accurately and volume for a required

thick layer is calculated. After fixing the density at which all tests are to be done, the

weight of the sand needed for a particular thickness of sand layer is calculated. The

density is found out to be 1.49 gm/cc and the layer thickness is 2.5 cm. For maintaining

the above density and layer thickness, required weight of sand is found out to be 13.78

kg. The box is filled by sand normally up to a depth of 30 cm and then rest 25 cm is filled

by using sand raining technique. The height of fall to achieve the required density is

determined by performing a series of trials with different height of fall. After filling of

each layer, leveling was done using a plane wooden plate and a level.

First the test is done without reinforcement and for the test with reinforcement; the first

geogrid layer is placed at a depth of 0.35B from the base of footing. The other subsequent

layer of geogrid is placed at equal spacing of 0.25B. After putting the geogrids, small

weights are placed on them to keep the geogrid in position and then the required weight

of sand is poured over it using sand raining technique. As the thickness of geogrid is very

small in comparison to the sand layer considered and grid has large openings, so it is

taken that the required density to be maintained is not affected.

3.7 TEST PROCEDURE

1. Upon filling the tank up to the desired height, the fill surface is leveled and the

footing is placed on a predetermined alignment such that the loads from the

cylindrical ram will be transferred vertically to the footing.

2. Then the LVDT indicator is placed at a suitable position on the footing to measure

the settlement of footing during the experiment. The LVDT digital indicator is set

to 80. The load cell indicator is set to 280.

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3. The static hydraulic loading system is switched on and the beam is moved up. The

four pins are removed and then the beam is moved down to the suitable position

and at this position the four pins are again inserted to keep the beam in locked

position during experimentation.

4. The cylindrical ram is moved down to place it exactly over the cross-hair marked

on the footing.

5. The NITAL software is started and a time limit is fixed to perform this

experiment. The load is applied on the footing by increasing the pressure.

6. The load on the footing and the corresponding settlement are noted after regular

intervals of time (say 5 min.).

7. The processes of load application is continued till there is failure of foundation

sand due to sudden excessive settlement, which can be observed in the Load cell

indicator where the load taken by the footing get decreased continuously.

8. On completion of the load test, he equipments are removed, box emptied and the

box again refilled for the next set of load test.

3.8 GEOMETRIC PARAMETERS

This fig shows a strip foundation of width ‘B’ supported on Geogrid reinforced sand.

There are four layers of geogrid, each having a width ‘b’. The top layer of geogrid is

located at a depth u below the bottom of the foundation. The distance between

consecutive layers of geogrid is ‘h’

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Here, B = width of foundation

L = length of foundation

u = Dist of top layer of reinforcement from the bottom face of foundation.

d = Depth of foundation

b = Width of each reinforcement layer

l = Length of each reinforcement layer

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d = u + (N-1) h

Where,

N – no. of reinforcement layers.

h – Vertical dist between 2 consecutive layers.

Hence, the total depth of reinforcement‘d’ bellow the bottom of the foundation is

d = u + (4-1) h

= u + 3h

In order to conduct model tests with geogrid reinforcement in sand, it is important to

decide the magnitude of u/B, b/B and h/B to derive maximum benefit in increasing the

ultimate bearing capacity. By conducting model tests on surface foundations supported

by sand with multiple layer of reinforcement, it was shown by several previous

investigators (Guido et al., 1987; Akinmusuru and akinbolande, 1981; yetimoglu et al.,

Shin and Das, 1990) that, for given values of h/B, u/B and b/B, the magnitude of BCRu

increases with u/B and attains a maximum value at (u/B)cr . For u/B > (u/B)cr, the

magnitude of BCRu decreases. By compiling several test results, Shin and Das (1999)

determined that (u/B)cr for strip foundations can vary between 0.25 to 0.50. Keeping all

these factors in mind, it is decided to adopt the following parameters for the present tests:

u/B = 0.35

h/B = 0.25

b/B = 4.50

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RESULTS AND DISCUSSIONS

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From the test results, the load intensity versus the settlement curves are plotted. In each

case, the ultimate bearing capacity is determined from the plotted graphs.

4.1 TESTS ON UNREINFORCED SAND

The test was conducted on unreinforced sand using 8cm width of footing. For vertical

loading condition, the ultimate bearing capacity, Qu of a strip foundation on unreinforced

sand is expressed by following two established theories.

1. Terzhagi theory

qu = 0.5γBNγ where Nγ = bearing capacity factor

Nγ = 2(Nq + 1) tanΦ

Nq = eπtanΦ tan2(π/4 + Φ/2)

Using the above relationship, the theoretical ultimate bearing capacity for the

present test conditions has been calculated.

Nγ = 185.44

qu = 1105.22 gm/cm2

Ultimate load taken by the footing = 318.304 kg

2. Meyerhoff theory

qu = 0.5γBNγ FγsFγdFγi

Fγs = 1+ 0.1(B/L) tan2(π/4 + Φ/2) = 1.114

Fγd = 1

Fγi = 1

Using the above relation it has been found out that –

qu = 996.85 gm/cm2

Ultimate load taken by the footing = 287.10 kg

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The experimental data required for the determination of qu (expt.) are given as below:

Table 4.1 Load intensity Vs Settlement (N =0)

Sl. No. Load Intensity (kg/cm2) Settlement (mm)

1 0 0

2 0.3 -0.27

3 0.5 -0.56

4 0.8 -0.90

5 0.973 -1.25

6 1.215 -1.65

7 1.469 -2.10

8 1.750 -2.75

9 2.058 -3.50

10 2.306 -4.30

11 2.554 -5.10

Load Intensity (kg/cm2)

-6

-5

-4

-3

-2

-1

00 1 2 3

B=8cm, N=0

Settl

emen

t (m

m)

qu(expt.) =1.4 kg/cm2

qu(the.) = 1.105 kg/cm2

Fig 4.1 Determination of qu (expt.) for unreinforced case of loading

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4.2 TESTS ON REINFORCED SAND

Tests were conducted on strip foundations supported on multilayered geogrid (BX1100)

reinforcements at various depths of below base of foundation (i.e. d/B = 0.6, 0.85, 1.10)

Huang and Menq (1997) have provided a tentative relationship to determine the ultimate

bearing capacity of a strip foundation on reinforced sand based on wide slab mechanism.

The relationships can be expressed as:-

quR = 0.5(B+ΔB) γNγ + γdNq

ΔB = 2d tanβ

tanβ = 0.68 – 2.071(h/B) + 0.743(CR) + 0.03(b/B) + 0.076N

CR is Cover Ratio = w/W = 0.107

w = width of longitudinal ribs

W = centre to centre spacing of longitudinal ribs

Nγ = 2(Nq + 1) tanΦ

Nq = eπtanΦ tan2(π/4 + Φ/2)

quR = 0.5(B+ΔB) γNγ + γdNq is valid in the following ranges only.

0 ≤ tanβ ≤ 1

0.25 ≤ h/B ≤ 0.5

0.02 ≤ CR ≤ 1.0

1 ≤ b/B ≤ 10

1 ≤ N ≤ 5

0.3 ≤ d/B ≤ 2.5

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Case 1:

d/B = 0.6, N = 2

tanβ = 0.5289

ΔB/B = 2(d/B) tanβ = 0.6347

quR = 2.253 kg/cm2

Case 2:

d/B = 0.85, N = 3

tanβ = 0.6049

ΔB/B = 1.028

quR = 2.908 kg/cm2

Case 3:

d/B = 1.10, N = 4

tanβ = 0.6809

ΔB/B = 1.498

quR = 3.639 kg/cm2

Case 4:

d/B = 1.35, N = 5

tanβ = 0.7569

ΔB/B = 2.0436

quR = 4.444 kg/cm2

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The experimental values and the corresponding load intensity vs settlement graph have

been obtained for the above mentioned conditions and are as follows:

Table 4.2 Load intensity Vs Settlement (N =2)

Sl. No. Load Intensity (kg/cm2) Settlement (mm)

1 0 0

2 0.4 -0.28

3 0.8 -0.85

4 1.2 -1.52

5 1.6 -2.20

6 2.0 -3.10

7 2.4 -4.50

8 2.8 -6.10

9 3.2 -8.00

10 3.6 -9.50

Fig 4.2Determination of qu (expt.) for reinforced case of loading, N = 2

Load Intensity (kg/cm2)

-12

-10

-8

-6

-4

-2

00 0.5 1 1.5 2 2.5 3 3.5 4

B=8cm, N=2, u/B=.35

Settl

emen

t (m

m)

qu(expt.)=2.38 kg/cm2

qu(the.)=2.253 kg/cm2

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Table 4.3 Load intensity Vs Settlement (N =3)

Sl. No. Load Intensity (kg/cm2) Settlement (mm)

1 0 0

2 0.4 -0.40

3 0.8 -0.72

4 1.2 -1.20

5 1.6 -1.70

6 2.0 -2.19

7 2.4 -2.90

8 2.8 -3.50

9 3.2 -4.20

10 3.6 -5.10

11 4.0 -5.80

12 4.4 -6.78

13 4.8 -7.70

14 5.2 -8.83

15 5.6 -9.80

16 6.0 -11.10

17 6.4 -12.40

Fig 4.3 Determination of qu (expt.) for reinforced case of loading, N = 3

Load Intensity (kg/cm2)

-14

-12

-10

-8

-6

-4

-2

00 1 2 3 4 5 6 7

B=8cm, N=3, u/B=.35

Settl

emen

t (m

m)

qu(expt.) = 3.45 kg/cm2

qu(the.) = 2.908 kg/cm2

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Table 4.4 Load intensity Vs Settlement (N =4)

Sl. No. Load Intensity (kg/cm2) Settlement (mm)

1 0 0

2 0.8 -0.9

3 1.6 -1.6

4 2.4 -2.8

5 3.2 -3.9

6 4.0 -5.2

7 4.8 -6.3

8 5.6 -7.7

9 6.4 -9.2

10 7.2 -10.6

11 8.0 -12.3

12 8.8 -14.0

13 9.6 -15.8

Fig 4.4 Determination of qu (expt.) for reinforced case of loading, N = 4

Load Intensity (kg/cm2)

-18

-16

-14

-12

-10

-8

-6

-4

-2

00 2 4 6 8 10 12

B=8cm, N=4, u/B=.35

Settl

emen

t (m

m)

qu(expt.)=4.22 kg/cm2

qu(the.)=3.639 kg/cm2

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Table 4.5 Comparison between experimental and theoretical BCRu

N d/B quR (the.) quR (expt.) BCRu(the) BCRu(expt)

2 0.6 2.253 2.380 2.04 1.70

3 0.85 2.908 3.450 2.63 2.46

4 1.10 3.639 4.220 3.29 3.01

Fig 4.5 Variation of BCRu with d/B

1

2

3

4

5

0.4 0.8 1.2 1.6

d/B

BC

Ru Series1

Series2

Series 1: theoretical value

Series 2: experimental value

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Table 4.6 Comparison of experimental values of quR with values calculated from equation

proposed by Huang and Menq, 1997.

N

quR (the.)

(kg/cm2)

quR (expt.)

(kg/cm2)

difference

(kg/cm2)

% difference

(kg/cm2)

2 2.253 2.380 0.127 5.64

3 2.908 3.450 0.442 15.20

4 3.639 4.220 0.681 18.71

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CONCLUSION

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The ultimate bearing capacity obtained from the tests has been compared with the

theoretical value developed by Huang & Menq, 1997. The following conclusions are

drawn from the tests conducted in the present study:-

1. The experimental load carrying capacity of a foundation on homogeneous sand or

on reinforced sand is always more than its theoretical load carrying capacity.

2. For the same soil, footing size and geogrid specification, the experimental and

theoretical values of ultimate bearing capacity increase and the settlement

increases with increase in number of geogrids.

3. The difference between experimental and theoretical values also increases with

increase in number of geogrid layer. The maximum % difference between the

experimental and theoretical values is 18.71%.

4. BCRu increases with increase in d/B ratio. From fig 4.5, it is concluded that BCRu

would reach a maximum at some d/B = (d/B)cr.

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SCOPE FOR FURTHER STUDIES

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Keeping in view of the limitations on time, available laboratory facilities and its scope of

present investigation, only a part of the problem was experimentally investigated. It is

necessary to investigate the ultimate load at failure and the corresponding settlement in

cohesive soil with geogrids as reinforcement.

Comprehensive investigation, both experimental and technical of the problem with

geogrid as reinforcement is desirable.

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REFERENCES

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1. Das, B.M., Omar, M.T., Shin, E.C. 2004. Developments on the bearing capacity of shallow foundations on geogrid-reinforced soil—a review. Proceedings, International Conference on Geotechnical Engineering, Sharjah, United Arab Emirates, 1-29.

2. Patra, C.R., Das, B.M., Bhoi, M., Shin, E.C. 2006. Eccentrically loaded strip foundation on geogrid-reinforced sand. Geotextiles and Geomembranes (in press).

3. Engineering with Geosynthetics – G.V Rao and G.V.S Raju. 4. Montanelli, F., Recalcati, P. Geogrid reinforced railways embankments: Design

concepts and experimental test results. 5. Designing with Geosynthetics – Robert M. Koerner 6. Principles of Geotechnical Engineering – B.M. Das 7. Huang, C.C., Menq F.Y. 1997. Deep footing and Wide-slab effects in reinforfced

sandy ground. Journal of Geotechnical and Geo environmental engineering.

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